In an image forming apparatus, when taking a 100% (life-size) copy, an image copied on a paper must be realized at a 100% magnification with respect to a document. However, a paper feeding speed and a drum circumferential speed coincide with each other due to a part dispersion of a carrier system, that is, a roller outer-diameter and a fixing roller outer-diameter; for this reason, a magnification error takes place. Moreover, in the case where there is no coincidence in a speed of each unit due to dispersion between oscillators of each motor control circuit, a magnification error takes place. Therefore, in the conventional image forming apparatus, the above 100% magnification is not always realized, and in order to realize the 100% magnification, a part accuracy of each unit must be strictly set, and a correction (adjustment) must be made while recognizing an actual copied image.
In a printer unit of a digital copying machine, a magnification error in its main scanning and feed has been corrected by varying a rotational speed of a polygonal motor and a rotational speed of a photosensitive drum motor (a paper feeding speed and a rotational speed of a photosensitive drum). On the other hand, in a scanner unit, a magnification error in its main scanning and feed has been corrected by varying a rotational speed of a polygonal motor and a scanner speed after a correction on the magnification error of the printer unit is made.
Inherently, in the case of making a correction on a magnification error in a main scanning direction, a lens position of a scanner unit and/or printer unit must be adjusted on the basis of an output signal of a CCD. However, an adjustment of a lens system is very troublesome; for this reason, the magnification error is corrected by a rotational speed of the polygonal motor. Moreover, although it is possible to correct the magnification error in a main scanning direction by image processing, this correction is not employed because there is the case where a disadvantage such as "moire" takes place.
According to the conventional magnification error correction in the main scanning/feed direction, in the case of an electronic sort using no finisher (in the case of image rotation), or in the case where a paper set in a transverse direction is run short out of a paper tray; for this reason, the paper tray is changed over from a paper try in which a paper is set in a longitudinal direction, the main scanning/feed direction in reading is replaced with a feed/main scanning direction in writing; as a result, a magnification error takes place. In order to reduce this magnification error, Jpn. Pat. Appln. KOKAI Publication No. 9-130582 has disclosed a method for correcting a magnification error in a longitudinal-transverse conversion. However, according to this method, in addition to magnification error correction in the feed and main scanning direction, each magnification error in a case of longitudinal-transverse conversion must be corrected.